It is known that the efficiency of a combustion turbine engine will improve as the firing temperature of the combustion gas is increased. As the firing temperatures increase, the high temperature durability of the components of the turbine must increase correspondingly. Although nickel and cobalt based superalloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes exceeding 1,400 degrees C. In many applications a metal substrate is coated with a ceramic insulating material in order to reduce the service temperature of the underlying metal and to reduce the magnitude of the temperature transients to which the metal is exposed.
Thermal barrier coating (TBC) systems are designed to maximize their adherence to the underlying substrate material and to resist failure when subjected to thermal cycling. The temperature transient that exists across the thickness of a ceramic coating results in differential thermal expansion between the top and bottom portions of the coating. Such differential thermal expansion creates stresses within the coating that can result in the spalling of the coating along one or more planes parallel to the substrate surface. It is known that a more porous coating will generally result in lower stresses than dense coatings. Porous coatings also tend to have improved insulating properties when compared to dense coatings. However, porous coatings will densify during long term operation at high temperature due to diffusion within the ceramic matrix, with such densification being more pronounced in the top (hotter) layer of the coating than in the bottom (cooler) layer proximate the substrate. This difference in densification also creates stresses within the coating that may result in spalling of the coating.
A current state-of-the-art thermal barrier coating is yttria-stabilized zirconia (YSZ) deposited by electron beam physical vapor deposition (EB-PVD). The EB-PVD process provides the YSZ coating with a columnar microstructure having sub-micron sized gaps between adjacent columns of YSZ material, as shown for example in U.S. Pat. No. 5,562,998. The gaps between columns of such coatings provide an improved strain tolerance and resistance to thermal shock damage. Alternatively, the YSZ may be applied by an air plasma spray (APS) process. The cost of applying a coating with an APS process is generally less than one half the cost of using an EB-PVD process. However, it is extremely difficult to form a desirable columnar grain structure with the APS process.
It is known to produce a thermal barrier coating having a surface segmentation to improve the thermal shock properties of the coating. U.S. Pat. No. 4,377,371 discloses a ceramic seal device having benign cracks deliberately introduced into a plasma-sprayed ceramic layer. A continuous wave CO2 laser is used to melt a top layer of the ceramic coating. When the melted layer cools and re-solidifies, a plurality of benign micro-cracks are formed in the surface of the coating as a result of shrinkage during the solidification of the molten regions. The thickness of the melted/re-solidified layer is only about 0.005 inch and the benign cracks have a depth of only a few mils. Accordingly, for applications where the operating temperature will extend damaging temperature transients into the coating to a depth greater than a few mils, this technique offers little benefit.
Special control of the deposition process can provide vertical micro-cracks in a layer of TBC material, as taught by U.S. Pat. Nos. 5,743,013 and 5,780,171. Such special deposition parameters may place undesirable limitations upon the fabrication process for a particular application.
U.S. Pat. No. 4,457,948 teaches that a TBC may be made more strain tolerant by a post-deposition heat treatment/quenching process which will form a fine network of cracks in the coating. This type of process is generally used to treat a complete component and would not be useful in applications where such cracks are desired on only a portion of a component or where the extent of the cracking needs to be varied in different portions of the component.
U.S. Pat. No. 5,558,922 describes a thick thermal barrier coating having grooves formed therein for enhance strain tolerance. The grooves are formed by a liquid jet technique. Such grooves have a width of about 100-500 microns. While such grooves provide improved stress/strain relief under high temperature conditions, they are not suitable for use on airfoil portions of a turbine engine due to the aerodynamic disturbance caused by the flow of the hot combustion gas over such wide grooves. In addition, the grooves go all the way to the bond coat and this can result in its oxidation and consequently lead to premature failure.
U.S. Pat. No. 5,352,540 describes the use of a laser to machine an array of discontinuous grooves into the outer surface of a solid lubricant surface layer, such as zinc oxide, to make the lubricant coating strain tolerant. The grooves are formed by using a carbon dioxide laser and have a surface opening size of 0.005 inch, tapering smaller as they extend inward to a depth of about 0.030 inches. Such grooves would not be useful in an airfoil environment, and moreover, the high aspect ratio of depth-to-surface width could result in an undesirable stress concentration at the tip of the groove in high stress applications.
It is known to use laser energy to cut depressions in a ceramic or metallic coating to form a wear resistant abrasive surface. Such a process is described in U.S. Pat. No. 4,884,820 for forming an improved rotary gas seal surface. A laser is used to melt pits in the surface of the coating, with the edges of the pits forming a hard, sharp surface that is able to abrade an opposed wear surface. Such a surface would be very undesirable for an airfoil surface. Similarly, a seal surface is textured by laser cutting in U.S. Pat. No. 5,951,892. The surface produced with this process is also unsuitable for an airfoil application. These patents are concerned with material wear properties of a wear surface, and as such, do not describe processes that would be useful for producing a TBC having improved thermal endurance properties.